Relict duck-billed dinosaurs survived into the last age of the dinosaurs in subantarctic Chile

In the dusk of the Mesozoic, advanced duck-billed dinosaurs (Hadrosauridae) were so successful that they likely outcompeted other herbivores, contributing to declines in dinosaur diversity. From Laurasia, hadrosaurids dispersed widely, colonizing Africa, South America, and, allegedly, Antarctica. Here, we present the first species of a duck-billed dinosaur from a subantarctic region, Gonkoken nanoi, of early Maastrichtian age in Magallanes, Chile. Unlike duckbills further north in Patagonia, Gonkoken descends from North American forms diverging shortly before the origin of Hadrosauridae. However, at the time, non-hadrosaurids in North America had become replaced by hadrosaurids. We propose that the ancestors of Gonkoken arrived earlier in South America and reached further south, into regions where hadrosaurids never arrived: All alleged subantarctic and Antarctic remains of hadrosaurids could belong to non-hadrosaurid duckbills like Gonkoken. Dinosaur faunas of the world underwent qualitatively different changes before the Cretaceous-Paleogene asteroid impact, which should be considered when discussing their possible vulnerability.


Supplementary Text
Taphonomy Different analyses were carried out including hydraulic equivalence, bone modification, assemblage data analyses and Voorhies groups classification. The overall study of taphonomic aspects followed traditional literature (102,103). Only bones of duck-billed dinosaurs were recovered at the site, as is common in the fossil record of these dinosaurs, probably related to their gregarious behavior (19). Most of the bones were found scattered without superposition and without any evident articulation. The size of the elements ranges between 4.8 cm and 48.1 cm, the former value corresponding to the length of the smallest caudal vertebral centrum (CPAP 5351) and the latter to the length of the largest femur (CPAP 5359). 44.5% of the elements are complete and 93% present some extent of fracture. A minimal number of three individuals (MNI) has been estimated based on the number of right femora and left humeri. Relative abundance of elements from each skeletal region is shown in Table S1.
The transport of a bone element in water depends on multiple factors, such as shape, density, size and degree of articulation, in addition to the flow velocity. Disarticulated bones form distinctive classification groups ("Voorhies groups") according to the flow speed to which they are subjected (104)(105)(106)(107). Group 1 corresponds to bones of lower density and relatively low mass, which are affected by light currents (vertebrae and phalanges, for example); group 2 is formed by elements that are eliminated gradually, mainly due to traction; and group 3 corresponds to bones that tend to resist transport (due to their greater density and size). We found a predominance of Group 3 elements, which is consistent with the interpretation of an original deposit with the influence of low stream current transportation (see Table S2). Fluvial transport is therefore the most likely explanation for the disarticulation and scattering of the elements within the quarry.
Hydraulic equivalence allows comparing the settling velocity of fossils with that of clasts to estimate whether they were deposited at similar flow velocities, that is, by a similar process (105). Assemblages that accumulated due to fluvial processes should have a high proportion of material in hydraulic equivalence with the sediment matrix, whereas a lack of hydraulic equivalence indicates that fluvial processes were not the dominant cause of accumulation (although it does not rule out fluvial influence, 108). The average length of (complete) bones is 22 cm; large bones range between 48 cm and 32 cm and represent 35% of the complete bones. The remaining 65% range between 24.6 cm and 4.8 cm in length. When comparing these dimensions with quartz grains, using the equivalence table of Behrensmeyer (105), most of the elements are found to be equivalent to gravel-sized grains. This means that the current velocity that transported these bones should have also been enough to transport gravel-sized grains. The matrix of the bonebed however consists predominantly of sandy mudstones with coal lenses and fine-grained sandstones. This discrepancy between the hydraulic equivalence of the bones and the matrix suggests that flow competence would have been insufficient to transport the bones a long distance, thus ruling out allochthonous assembly. This result is consistent with the interpretation based on the Voorhies group classification.
Analysis of pre-fossilization and diagenetic weathering are precluded due to current climatic conditions at the site (i.e., snow cover during most of the year, temperature fluctuations and strong winds during the summer) which have an important influence on the state of preservation, accelerating weathering and erosion.
Abrasion stages can be represented by a number from 0 to 3 where 0 represents pristine fossil surfaces without signs of abrasion, and 3 corresponds to fossil bones with extremely wellrounded edges (106,109). The elements at the bonebed show low levels of abrasion (84.5% with stage 0 and 15.5% with stage 1) that suggests minimal transport for most specimens. As already mentioned, practically all the bones are incomplete. 93.3% of the elements present fractures, most of which correspond to transverse and longitudinal fractures as caused by trampling, or contemporary exposure of the fossil material to temperature variations (climate) that produce the expansion and contraction of the material. No spiral fractures have been found in the analyzed fossils.
The different analyses above support the idea that the elements were either not transported or underwent minimal transport. This leads us to conclude that the bones are close to the original thanatocoenosis and probably to the habitat of these animals, being parauthoctonous and synchronic to some degree. The surface of many of the bones is damaged due to their exposure to current extreme weather conditions, precluding an appropriate bone modification analysis. The present data rule out death by predation and scavenging since there are no bite marks on the elements and no predator teeth have been recovered in the quarry. All lines of taphonomic evidence indicate that the reasons for the death of the individuals are biological. Considering this interpretation, the dominance of elements from Voorhies Group III is unusual. However, it may be the result of a flooding event, which would have removed elements from Voorhies Groups 1 and 2 at a time when the skeletons were exposed.
List of rescored and redefined characters in the data set of Rozadilla et al. (4) 1) Secernosaurus koerneri (character 258). Changed from 1 to ?, since the distal half of the scapula is not preserved.

Reconstruction of biogeographic history
The results of the BioGeoBEARS analysis are summarized in Table S4 and Fig. S11. The best-fit model in BioGeoBEARS was the DIVALIKE + j model (AICc: 200.80), which supports a Laramidian origin for the last ancestor shared by Gonkoken with Hadrosauridae, and for the last ancestor shared with Eotrachodon, at the previous node (Fig. S11). The + j versions of all models (DEC, BAYERALIKE, and DIVALIKE) showed a better fit to the data and supported a Laramidian origin for the last ancestor shared by Gonkoken with Hadrosauridae, and for the last ancestor shared with Eotrachodon. The + j models support a strong role of founder effects (114), which is consistent with discrete events of dispersal across important barriers as proposed for Hadrosauroidea.
In s-DIVA, the last ancestor shared by Gonkoken and Hadrosauridae is recovered as having inhabited either Laramidia + Appalachia + South America, or Appalachia + South America, with equal probability. In turn, the previous last ancestor shared by Gonkoken and Eotrachodon inhabited Appalachia rather than Laramidia, as inferred by BioGeoBEARS (Fig. S12). Despite these differences, the results of s-DIVA are generally consistent with those of BioGeoBEARS in that they support the arrival of the ancestors of Gonkoken from North America. Possible reasons for the differences between the results of s-DIVA and BioGeoBEARS are that only BioGeoBEARS considers the geological age of taxa in time-calibrated trees, and that BioGeoBEARS includes a dispersal matrix that considers different dispersal probabilities between land masses.

Conceptual experiments with a hypothetical African taxon
When the Hypothetical African taxon was placed between Eotrachodon and Gonkoken, the best model (DIVALIKE + j; AICc: 218.83, see Table S5) supported Laramidia + South America as the ancestral area for the last ancestor that Gonkoken shared with Hadrosauridae, and Laramidia as the most likely area for the last ancestor shared with Eotrachodon (Fig. S13).
When a Hypothetical African taxon was added to our BioGeoBEARS analysis as sister taxon to Gonkoken, the best fitting model (BAYAREALIKE + j; AICc: 210.89, see Table S6) did not show significant changes and continued to support Laramidia as the ancestral area for both the last ancestor that Gonkoken shared with Hadrosauridae, and the last ancestor shared with Eotrachodon (Fig. S15).